1. Field of the Invention
The present invention generally relates to high-efficiency insulation products (e.g., panels) and, more particularly, to high-efficiency insulation products that may be applied about cylindrically-shaped or other non-planar objects (e.g., pipes, tanks, etc.) to limit heat transfer into and out of the non-planar objects.
2. Relevant Background
Thermal insulation generally refers to a porous material with an inherently low thermal conductivity serving to protect the system of interest from heat flow into or out of its surroundings. The use of thermal insulation is prevalent in society ranging from use in domestic refrigerators (e.g., for reduced energy consumption or additional internal volume), in shipping containers containing ice or dry ice used for drugs or food (e.g., to extend the lifetime of the shipment), in the tiles on the space shuttle (e.g., used to protect the shuttle from the heat of reentry into the atmosphere), and/or the like. Most thermal insulation products used today are either fibrous materials, such as fiberglass, mineral wool and asbestos, or polymer foams, such as expanded polystyrene, polyurethane, foamed polyethylene and foamed polypropylene. Use of fibrous materials may be undesirable in many instances due to problems related to health and safety. Use of polymer foams may be undesirable due to their flammability, lack of recyclability and release of environmentally unfriendly gases, such as fluorocarbons or hydrocarbons during manufacture. In addition, the thermal performance of both fibrous materials and polymer foam materials are on the same order as or greater than stagnant air (e.g., about 0.026 W/mK at ambient temperature). Because of increased concern with respect to energy efficiency and the environment, there has been much interest in the development of new classes of thermal insulation that have a thermal conductivity much less than that of air, such as aerogels, inert gas-filled panels and vacuum insulation panels.
For thermal insulation, a key measure of performance is the thermal conductivity of the material. More specifically, lower thermal conductivity means lower heat flow through the insulation for a given temperature difference. In the absence of convection, heat transfer through insulation occurs due to the sum of three components: solid phase conduction, gas phase conduction and radiation. Solid phase conduction may be minimized by using a low density material (e.g., a material comprising a high volume fraction of pores). Most insulation is between, for instance, 80 and 98% porous. It is also advantageous to use a solid material that has a low inherent thermal conductivity (e.g., plastics and some ceramics/glasses are better than metals).
The relative importance of radiation depends upon the temperature range of interest and becomes a more prevalent component as the temperature is increased above ambient and/or the magnitude of the other heat transfer modes is minimized. Materials with high infrared (IR) extinction coefficients due to absorption (e.g., IR opacifiers such as carbon black, iron oxide, etc.) or scattering (e.g., titania) are often added to high performance insulation to limit radiative heat transfer.
With control of radiation, suppression of free convection, use of low thermal conductivity materials and a highly porous solid matrix, the thermal conductivity of the insulation approaches that of the gas contained within the pores of the insulation. There are a number of methods for lowering gas phase conduction in insulation. One method to do so is to trap gases in the pores that have lower thermal conductivity than that of air, such as argon, carbon dioxide, xenon and krypton. Depending upon the gas employed, the thermal conductivity of insulation filled with an inert gas can range from, for instance, 0.009 to 0.018 W/mK. However, the insulation must be packaged such that the filler gas does not leak from the pores and also so that atmospheric gases (e.g., nitrogen, oxygen) do not penetrate the insulation.
Another method for controlling or lowering gas phase conduction is to employ the Knudsen effect. Generally, gas phase conductivity within the insulation may be dramatically reduced when the mean free path of the gas approaches the pore size of the insulation. In fact, gas phase conductivity may approach zero (so that the total effective thermal conductivity is the sum of only radiation and solid phase conduction) when the mean free path of the gas is much larger than the pore size. For instance, the mean free paths of the components of air are approximately 60 nanometers at ambient temperature and pressure, while the pore/cell size of polymer foams and fibrous materials is typically greater than 10 microns.
There are at least two approaches that can employ the Knudsen effect to lower gas phase conduction. A first approach is to encapsulate the insulation within a barrier material and partially evacuate the gas in the insulation (i.e., use a vacuum pump to evacuate the insulative material). This increases the mean free path of the gas by lowering the gas density, which lowers gas phase conduction. Materials employing such gas evacuation techniques can achieve a thermal conductivity of less than 0.002 W/mK at ambient temperatures, which is an order of magnitude improvement over conventional insulation.
The advantages of utilizing a vacuum with an insulative material have been known for many years and are the basis of vacuum Dewars that are used with cryogenic liquids and for storing hot or cold beverages or other products. For example, U.S. Pat. No. 1,071,817 by Stanley discloses a vacuum bottle or Dewar, where a jar is sealed inside another jar with a deep vacuum maintained in the annular space with the two jars being joined at the jar mouth. Such an approach minimizes joining and thermal bridging problems, but most insulation applications require many different shapes that cannot be met by a Dewar.
Another approach is to use a material with very small pores and low density. One such class of materials is nanoporous silica, also known as silica aerogels, which generally have small pores (e.g., <100 nm), a low density, and exhibit a total thermal conductivity at ambient pressure that is lower than that of the gas contained within the pores. It is known to use nanoporous silica in conjunction with a vacuum to create a vacuum insulation panel (VIP). U.S. Pat. No. 4,159,359 by Pelloux-Gervais discloses the use of compacted silica powders, such as precipitated, fumed, pyrogenic, or aerogels, contained in plastic barriers, which are subsequently evacuated and then sealed.